A dissolution method known from the prior art, and illustrated in FIGS. 1A and 1B, is a method for dissolving a silicon dioxide layer in a structure 5 of the semiconductor-on-insulator type, comprising, from its rear face 6 to its front face 7, a supporting substrate 8, the silicon dioxide layer 9 and a semiconductor layer 10. The front face 7 corresponds to the free surface of the semiconductor layer 10.
A person skilled in the art will find a technical description of such a method in the articles by Kononchuk (Kononchuk et al., “Novel trends in SOI technology for CMOS applications,” Solid State Phenomena, Vols. 156-158 (1010) pp. 69-76, and Kononchuck et al., “Internal Dissolution of Buried Oxide in SOI Wafers,” Solid State Phenomena, Vols. 131-133 (2008) pp. 113-118).
This dissolution method may be implemented in a furnace 1, illustrated in FIG. 2, in which a plurality of structures 5 are held on a support 4 so that the support 4 is suitable for holding the structures 5 with a predetermined distance, typically a few millimeters, between each structure 5, the front face 7 of a structure 5 being opposite the rear face 6 of the structure 5 adjacent to the front face 7.
The structures 5 are subjected to a non-oxidizing atmosphere. The non-oxidizing atmosphere is provided by a continuous flow of inert or reducing gas. The flow of inert gas enters the furnace 1 through an inlet 2, and emerges therefrom through an outlet 3.
The use of such a heat treatment causes the diffusion of oxygen atoms included in the silicon dioxide layer 9 through the semiconductor layer 10. The reaction of the oxygen atoms with the semiconductor layer 10 generates volatile products comprising semiconductor monoxide (ScO). However, the gases present on the surface of the structures 5, in particular, the volatile products generated, have an influence on the dissolution.
Thus, the semiconductor monoxide slows the dissolution reaction when its concentration on the surface of the structures 5 increases.
The composition of the atmosphere of the furnace 1 is not homogeneous. This is because of the small spacing between the structures 5. The volatile products are discharged solely by diffusion at the edge of the structures 5. The result of this is an accumulation of the volatile products that is greater at the center of the surfaces of the structure 5 than at their edge. This means that the dissolution reaction is more rapid at the periphery than at the center of the structures 5.
Moreover, the atmosphere of the furnace 1 is obtained by a constant flow of an inert or reducing gas. The flow of gas entrains, from its entry at inlet 2 into the furnace 1 to its discharge at outlet 3, at least some of the volatile products. Consequently, during its path through the furnace 1, the flow of gas becomes loaded with volatile products.
Depending on their location in the furnace 1, the structures 5 are, therefore, subjected to a variable concentration of volatile products.
Finally, the flow of gas may contain small quantities of oxygen.
Since a complete absence of oxygen in the gas flow would require using very complex means, a small percentage of oxygen in the gas flow entering the furnace 1 is tolerated.
The oxygen included in the atmosphere of the furnace 1 limits the dissolution of the layer of silicon dioxide 9 and degrades the roughness of the free surface of the semiconductor layer 10.
The oxygen contained in the gas flow reacts preferentially with the structures 5 close to the gas inlet 2. The gas flow is, therefore, depleted of oxygen from the inlet 2 of the furnace 1 toward the outlet 3.
This non-homogeneity of the atmosphere of the furnace 1 results in significant variabilities on the characteristics of the structures 5.
The main drawback of this dissolution method is that the non-homogeneity of the atmosphere of the furnace 1 leads to a degradation in the uniformity of thickness of the silicon dioxide layer 9 and of the semiconductor layer 10, as illustrated in FIG. 1B. This is because, at the end of the heat treatment, the thickness of the silicon dioxide layer 9 and the thickness of the semiconductor layer 10 are greater at the center than at the edge of the structure.
Another drawback of this dissolution method is that it is not uniform for all the structures 5 of the semiconductor-on-insulator type contained in the furnace 1. This is because the silicon dioxide layer 9 is not dissolved in the same proportions from one structure 5 to another.
The aforementioned drawbacks are not observed in a furnace 1 containing a single structure 5. However, given the relatively long heat treatment times and for economic reasons, executing such a method in a furnace 1 containing only one structure 5 cannot be envisaged from an industrial point of view.
However, some applications require having recourse to a silicon dioxide layer 9 with a thickness of less than 50 nm so as to be able to apply, for example, an electrical voltage exerted on devices produced in or on the semiconductor layer 10. A very precise control of the thickness of the silicon dioxide layer is then necessary.
Moreover, the structures 5 designated by the term “FDSOI,” standing for “fully depleted silicon-on-insulator,” are particularly advantageous for producing electronic components such as FDMOS (“fully depleted metal oxide semiconductor”) transistors, the channel of which is formed in or on the semiconductor layer 10.
Because of the extreme fineness of the thickness of the semiconductor layer 10 (i.e., around 10 nm), the threshold voltage of the transistor (usually denoted Vt), which depends on this thickness, is very sensitive to the variations in thickness of the semiconductor layer 10.
One aim of the disclosure is, therefore, to propose a method for dissolving a silicon dioxide layer affording precise control of the thicknesses of the semiconductor and silicon dioxide layers.